Here is the full list of all 12 Amatarasu Riddle Star achievements.
Complete Rina's mini-route.
Complete the game.
Complete Hire's mini-route.
Complete Melody's mini-route.
Complete Miu's route.
Complete Ai's route.
Complete Kokoro's route.
Complete Yua's route.
Complete Madori's route.
Choose someone to spend time with.
Finish preparations for the school festival.
Complete the sports festival.